we don't quite know, but i don't think the president would have made this comment and sen secretary clinton afterwards without there being some pretty serious indicators that we were getting close to possible use. >> brown: one of the things we're hearing is the potential for mixing compounds. what would that mean? >> at least the sarin is thought to be in two parts, the binary weapon, and it has to be mixed together prior to the time the weapon is actually used. sometimes the mixing occurs wihin the weapon iself, and sometimes the mixing occurs beforehand and the weapon is filled with the actual dangerous, lethal chemical agent. >> brown: now if this is said to be one of the world's largest stockpiles, how hard is it to stop, or how hard is it to deal with? would we even know if he was ready to use it? >> that's one of the big problems. you might not know precisely how close he was to using them and it might be very hard to head this off at the moment of imminence, just before it was about to occur. if you try to do it at that moment, you may actually cause the chemicals themselveses to ex